If you select someone based upon reviews, then you will get an idea of how legitimate she is.  You should be able to choose a "legitimate" provider, this means that it should be safe to ask her in general and polite terms about her policies on the items that you listed as concerns.  If you avoid being graphic with your questions a professional should not have any problem giving you answers upfront.
    There are many people here that are members of this site that have been involved in the use of oral dams and female condoms.  Both should allow your girlfriend to protect herself and also avoid having her tongue or lips directly on a vagina.  You should try posting a question about oral dams and female condom on the General discussion board, there seem to be a large number of people posting on that board that have experience with one or both.

Most every provider loves to take a shower with their client(s), so if you have a big enough shower, imagine getting washed down by both your GF and the provider.  To me that would probably be more fun than the actual main event.

Just put the one being washed in the middle, one person washes the back and the other the front of the washed one.  Then swap two more times.

Then you will all be squeeky clean!